ABSTRACT
The intrinsic heterogeneity of natural porous media, with regard to physical and chemical properties, gives rise to complicated transport behaviour of reactive chemicals. For an adequate description of transport, first the scale of interest needs to be defined. At this scale, the transport equations are formulated and smaller scale heterogeneity needs to be treated adequately, e.g. by averaging, but it is not taken explicitly into account. Small scale heterogeneity may give rise to e.g. Freundlich type adsorption that is locally not in equilibrium. For a one-dimensional flow domain, this may induce traveling wave type of displacement, provided certain conditions are met. These conditions are indicated. Spatial variability of properties and boundary conditions in the horizontal direction in the vadose zone may be dealt with using either impulse-response functions or stochastic methods. Use of both approaches (until 1990) is addresses. In particular, spatial variability with regard to both physical and chemical properties-appears to be important. Chemical heterogeneity appears to be relatively underexposed in the literature. Different types of heterogeneity may give rise to fingering phenomena in the unsaturated zone. For reactive solutes the persistence of fingers as well as the travel time distributions originating form this type of flow pattern may be of main importance with regard to solute leaching into groundwater. In groundwater, the main direction of flow is generally parallel to stratification. The loss of solute by diffusion into low permeable layers depends on dispersive properties of different layers, as well as differences in retarded velocities. Different recent advances for perfectly stratified media and transport parallel to layering indicate that transversal losses may be significant. Whereas profound advances have been made in solute transport modeling, both theoretical and experimental research concerning the interaction of (bio)chemical aspects on transport behaviour in natural porous media is needed.
